1. Field of the Invention
The present invention relates to a family of power transmissions having three planetary gear sets that are controlled by six torque-transmitting devices to provide at least seven forward speed ratios and one reverse speed ratio.
2. Background Art
Passenger vehicles include a powertrain that is comprised of an engine, multi-speed transmission, and a differential or final drive. The multi-speed transmission increases the overall operating range of the vehicle by permitting the engine to operate through its torque range a number of times. The number of forward speed ratios that are available in the transmission determines the number of times the engine torque range is repeated. Early automatic transmissions had two speed ranges. This severely limited the overall speed range of the vehicle and therefore required a relatively large engine that could produce a wide speed and torque range. This resulted in the engine operating at a specific fuel consumption point during cruising, other than the most efficient point. Therefore, manually-shifted (countershaft transmissions) were the most popular.
With the advent of three- and four-speed automatic transmissions, the automatic shifting (planetary gear) transmission increased in popularity with the motoring public. These transmissions improved the operating performance and fuel economy of the vehicle. The increased number of speed ratios reduces the step size between ratios and therefore improves the shift quality of the transmission by making the ratio interchanges substantially imperceptible to the operator under normal vehicle acceleration.
It has been suggested that the number of forward speed ratios be increased to five and six speeds. This has been accomplished in many heavy truck powertrains. Six-speed transmissions are disclosed in U.S. Pat. No. 4,070,927 issued to Polak on Jan. 31, 1978; U.S. Pat. No. 6,071,208 issued to Koivunen on Jun. 6, 2000; U.S. Pat. No. 5,106,352 issued to Lepelletier on Apr. 21, 1992; and U.S. Pat. No. 5,599,251 issued to Beim and McCarrick on Feb. 4, 1997.
Six-speed transmissions offer several advantages over four- and five-speed transmissions, including improved vehicle acceleration and improved fuel economy. While many trucks employ power transmissions, such as Polak, having six or more forward gear ratios, passenger cars are still manufactured with three- and four-speed automatic transmissions and relatively few five or six-speed devices due to the size and complexity of these transmissions. The Polak transmission provides six forward speed ratios with three planetary gear sets, two clutches, and three brakes. The Koivunen and Beim patents utilize six torque-transmitting devices including four brakes and two clutches to establish six forward speed ratios and a reverse ratio. The Lepelletier patent employs three planetary gear sets, three clutches and two brakes to provide six forward speeds. One of the planetary gear sets is positioned and operated to establish two fixed speed input members for the remaining two planetary gear sets.
Seven-speed transmissions are disclosed in U.S. Pat. No. 4,709,594 to Maeda; U.S. Pat. No. 6,053,839 to Baldwin et. al.; and U.S. Pat. No. 6,083,135 to Baldwin et. al. In addition to further improvements in acceleration and fuel economy, seven- and eight-speed transmissions provide an attractive marketing feature. However, like the six-speed transmissions discussed above, the development of seven- and eight-speed transmissions has been precluded because of complexity, size and cost.
It is an object of the present invention to provide an improved family of transmissions having three planetary gear sets controlled to provide at least seven forward speed ratios.
In one aspect of the present invention, the family of transmissions has three planetary gear sets, each of which includes a first, second and third member, which members may comprise a sun gear, a ring gear, or a planet carrier assembly member.
In another aspect of the present invention, each of the planetary gear sets may be of the single pinion-type or of the double pinion-type.
In yet another aspect of the present invention, the first member of the first planetary gear set is continuously interconnected to the first member of the second planetary gear set through a first interconnecting member.
In still another aspect of the invention, a second interconnecting member continuously interconnects the second member of the first planetary gear set with the first member of the third planetary gear set.
In yet another aspect of the invention, a third interconnecting member continuously interconnects the second member of the second planetary gear set with the second member of the third planetary gear set.
In yet a further aspect of the invention, each family member incorporates an input shaft which is continuously interconnected with at least one member of the planetary gear sets and an output shaft which is continuously connected with another member of the planetary gear sets.
In still a further aspect of the invention, a first torque-transmitting mechanism, such as a brake, selectively interconnects the stationary transmission housing with a member of the first or second planetary gear sets.
In another aspect of the invention, a second torque-transmitting mechanism, such as a brake, selectively interconnects the stationary transmission housing with the first, second or third interconnecting member or a member of the second or third planetary gear sets.
In a still further aspect of the invention, a third torque-transmitting mechanism, such as a brake, selectively interconnects a member of the first, second or third planetary gear set with the transmission housing.
In a still further aspect of the invention, a fourth torque-transmitting mechanism, such as a clutch, selectively interconnects a member of the first or second planetary gear set with the input shaft, the output shaft, or another member of the first, second and third planetary gear sets.
In still another aspect of the invention, a fifth torque-transmitting mechanism, such as a clutch, selectively interconnects a member of the second or third planetary gear sets with one of the first, second or third interconnecting members, or another member of the first, second or third planetary gear sets.
In still another aspect of the invention, a sixth torque-transmitting mechanism, such as a clutch, selectively connects a member of the first, second or third planetary gear sets with another member of the first, second or third planetary gear sets. Alternatively, the sixth torque-transmitting mechanism is a brake which selectively connects a member of the first, second or third planetary gear sets or one of the first, second or third interconnecting members with the transmission housing.
In still another aspect of the invention, the six torque-transmitting mechanisms are selectively engageable in combinations of two to yield at least seven forward speed ratios and one reverse speed ratio.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings.